This project is a study of the development of anisotropic visual sensitivity--the ability to see narrow contours on some orientations more clearly than on others, even when there is no opticla error. Its specific aims are 1) to document the course of visual sensitivity changes during childhood which eventually lead to greater sensitivity for horizontal and vertical contours than for diagonal ones in Western adults; and 2) to determine the variables which are responsible for such changes in sensitivity. Two courses of investigation are proposed here. One is to develop more efficient, criterion-free testing techniques for 4- to 6-year-old children. The second is to test three hypotheses about what is changing when adults eliminate their anisotropy for a diagonal by practicing detecting that diagonal, and to investigate the specificity and the persistence of the sensory improvements that result from practice. Two methods will be developed for testing yound children. One is a two-alternative force-choice (2AFC) paradigm which is incorporated into a computer-controlled video game with modified staircasing of the contrast or spatial frequency of the test patterns. Such a method should have wide application to other areas of developmental psychophysics, as well as to the study of the development of anisotropic contrast sensitivity and acuity. The second method uses frequency-swept high-contrast sinusoidal gratings to evoke brain potentials (EPs). Pilot work shows that such a method is not only quick, but it is sensitive enough to pick up 5 c/deg differences in acuity as a function of orientation in adults. Signal detection, 2AFC and EP methods will be used to test hypotheses about the source of the sensory improvements that result from practicing a diagonal. The three hypotheses to be discriminated are that practice 1) stabilizes accommodation, 2) improves knowledge about where to look in orientation space for the stimulus, or 3) increases the number of visual cortical cells processing information for the practiced orientation. This research not only provides information about the plasticity of the adult human visual system, but also may prove useful in devising therapies for meridional amblyopes.